A team of researchers led by the University of California San Diego has just identified the bacterial toxin colibactin as a potential colorectal cancer culprit. The findings, published in the journal Nature, revealed that childhood exposure to this toxin, which is produced by certain strains of the bacteria E. coli, could cause mutations in the DNA of the colon that increase the risk of developing colorectal cancer before age 50.

“When we started this study, we weren’t looking to answer this specific question,” says Ludmil Alexandrov, lead author of the study and professor in the Department of Cellular and Molecular Medicine at UC San Diego and a member of UC San Diego Moores Cancer Center. Initially, Alexandrov’s team was looking more broadly at colorectal cancer—when they discovered a difference in cancer cells among younger and older patients.

“We were very surprised to see that there was a bacteria, and that bacteria was generating mutations,” he tells Fortune. 

What is colibactin?

Colibactin is a toxin that can mutate DNA in colon cells, Alexandrov says, and those mutations can leave people more susceptible to cancerous cell growth. When children are exposed to the toxin, their DNA can go through thousands of mutations, Alexandrov says. That starts the process of potential cancer development at an even earlier age, increasing the chances that a mutation will lead to colorectal cancer.

His team found that colibactin leaves behind specific patterns of DNA mutations that were 3.3 times more common in early-onset colorectal cancer cases in adults under 40, compared to those diagnosed after age 70. They also found that more than 50% of the cancerous mutations across all ages were from this bacteria.

How are you exposed to colibactin?

As the study points out, the toxin colibactin is found in certain strains of E. coli—a group of bacteria that can cause infections in your gut, urinary tract, and other areas of your body.

Specific strains of E. coli can make you sick, but Alexandrov says that many of us are likely living with the bacteria in our systems and don’t know it, since it doesn’t always cause symptoms. The kind of E. coli that does get you sick with diarrhea, which is what most people likely think of, comes from fecal-oral transmission, undercooked or raw meat, unclean fruits and vegetables, or unpasteurized milk, cider, or juice, according to the Cleveland Clinic.

But because we can have E. coli in our systems without knowing, it’s tough to know if the toxin colibactin entered your gut or not. Being exposed to colibactin, however, does not guarantee you will develop cancer.

“The fact that you have it doesn’t mean it’s going to mutate,” Alexandrov says.

Why does this affect younger adults more?

Alexandrov hypothesizes that certain changes for younger generations have created more opportunities for exposure to E. coli and colibactin. 

Being born through a cesarean section—which is becoming more common—versus a vaginal birth may put a child at greater risk of contracting the bacteria because of resulting differences in how babies’ gut microbiomes—the ecosystem of microbes and bacteria that live in your intestines—develop.

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Additionally, Alexandrov and his colleagues believe that the increased use of antibiotics, the decline in breastfeeding, and increased consumption of ultraprocessed foods can all impact children’s gut health and immunity, leaving them more susceptible to the bacteria.

Children could be more exposed to this bacteria, leaving them more opportunity to develop the cancer at a younger age. While for older adults the first mutations might normally start at 35, Alexandrov says, for some that first mutation is starting at age 5, leaving more and more opportunity for cancer to develop.

The good news

Although millennials and Gen Xers can’t know for sure if they are harboring this bacteria in their gut, this discovery could provide vital insight into early cancer detection. Using this information, Alexandrov is hoping to develop a stool test to see if people have had the infection and mutations from this toxin. 

A stool sample, he explains, would contain DNA from colon tissue to help medical providers see evidence of mutations—the more mutations, the higher someone’s risk of colorectal cancer. Once you know someone is at high risk, you can take greater steps to monitor and catch the cancer early.

“If you capture colorectal cancer at its early stage, it’s very curable,” Alexandrov says.

But that test likely would take at least a couple of years to develop, he adds. In the meantime, this study can help people be more aware that this could happen—and not to ignore any potential symptoms, no matter your age.

“Having that awareness, and if you think something is wrong, you should go and have it checked out,” Alexandrov says. “I think that’s the best someone can do at this point.”

“This research tells us that things that happen in our childhood may have an impact long-term,” he says. “So things that happen to somebody in the first four, five years of life can affect their thirties and forties.”

Alexandrov and his team suspect that this might be true for other types of cancer, or other diseases as well. “We usually do not connect these childhood events to adulthood diseases, but it’s something for parents to be aware of,” he says.

For more on colorectal cancer:

• Vitamin D supplements may lower your risk of colorectal cancer by as much as 58%, according to a new study
• More millennials and Gen Xers are getting colorectal cancer. Adding these foods to your diet may help
• Working out at these times of day could reduce your risk of colorectal cancer by 11%, new study says
• Study confirms link between cancer and at least 10 popular ultraprocessed foods